This disclosure relates generally to a scanning system for detecting defects in a chargeable surface. More particularly, this disclosure relates to a contactless system and method for detecting defective points on a chargeable surface.
Although the concept of this disclosure includes any type of system for constant distance, contactless scanning of chargeable surfaces used in diverse applications, such as charge sensing probes for xerography, print heads for ink jet printing, ion stream heads for ionography, extrusion dies for coating, LED image exposure bars, and the like, the following discussion is directed to prior art systems for scanning chargeable surfaces used in xerography for illustrative purposes.
In the art of xerography, a xerographic plate or photoreceptor having a photoconductive insulating layer is provided. An image is acquired by first uniformly depositing an electrostatic charge on the imaging surface of the xerographic plate and then exposing the plate to a pattern of activating electromagnetic radiation, such as light, which selectively dissipates the charge in the illuminated areas of the plate while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the imaging surface.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium, or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in electrophotography is described in U.S. Pat. No. 4,265,990, the entire disclosure thereof being incorporated herein by reference. The patent describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photo-generating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are positioned on an electrically conductive layer with the photoconductive layer sandwiched between a contiguous charge transport layer and the conductive layer, the outer surface of the charge transport layer is normally charged with a uniform electrostatic charge, and the conductive layer is utilized as an electrode. In flexible electrophotographic imaging members, the electrode is normally a thin conductive coating supported on a thermoplastic resin web.
The conductive layer may also function as an electrode when the charge transport layer is sandwiched between the conductive layer and a photoconductive layer which is capable of photogenerating electrons and injecting the photogenerated electrons into the charge transport layer. The charge transport layer in this embodiment must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
The photoreceptors are usually multilayered and comprise a substrate, an optional conductive layer (if the substrate is not itself conductive), an optional hole blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer and, in some belt embodiments, an anti-curl backing layer.
In a photoreceptor, many types of microdefects can be a source of xerographic image degradation. These microdefects can be occlusions of particles, bubbles in the coating layers, microscopic areas in the photoreceptor without a charge generator layer, coating thickness non-uniformities, dark decay non-uniformities, light sensitivity non-uniformities, and charge deficient spots (CDSs). These last types of defect, charge deficient spots (CDSs) are localized areas of discharge without activation by light. They can cause two types of image defects, depending on the development method utilized. Charge deficient spots usually can be detected electrically or by xerographic development. They typically elude microscopic or chemical detection.
In discharged area development, the photoreceptor is negatively charged. An electrostatic latent image, as a charge distribution, is formed on the photoreceptor by selectively discharging certain areas. Toner attracted to discharged areas develops this latent image. Laser printers usually work on this principle. When charge deficient spots are present on the photoreceptor, examination of the final image after toner transfer form the photoreceptor to a receiving member, such as paper, reveals dark spots on a white background due to the absence of negative charge in the charge deficient spots.
In charged area development, usually used in light lens xerography, the toner image is formed by developing the charged areas on a photoreceptor. After transfer of the toner image to a receiving member, such as paper, the charge deficient spot on the photoreceptor results in a small white spot in a black background called a microwhite, which is not as noticeable as a “microblack” spot, characteristic of discharged area development.
One technique for detecting charge deficient spots in photoreceptors from a specific production run is to cycle the photoreceptor in the specific type of copier, duplicator and printer machine for which the photoreceptor was fabricated. Generally, it has been found that actual machine testing provides the most accurate way of detecting charge deficient spots in a photoreceptor from a given batch.
However, machine testing for detecting charge deficient spots is a laborious and time consuming process involving hand feeding of sheets by test personnel along with constant monitoring of the final quality of every sheet. Moreover, accuracy of the test results depends a great deal upon interpretations and behavior of the personnel that are feeding and evaluating the sheets.
Further, since machine characteristics vary from machine to machine for any given model or type, reliability of the final test results for any given machine model must factor in peculiar quirks of that specific machine versus the characteristics of other machines of the same model or type. Because of machine complexity and variations from machine to machine, the data from a test in a single machine is not sufficiently credible to justify the scrapping of an entire production batch of photoreceptor material.
Thus, tests are normally conducted in three or more machines. Since a given photoreceptor may be used in different kinds of machines such as copiers, duplicator and printers under markedly different operating conditions, the charge deficient spots detection based on the machine tests of a representative test photoreceptor sample is specific to the actual machine in which photoreceptors from the tested batch will eventually be utilized. Thus, photoreceptor tests on one machine do not necessarily predict whether the appearance of charge deficient spots occur if the same type of photoreceptor were used in a different type of machine.
Thus, for a machine charge deficient spot test, the test would have to be conducted on each different type of machine. This becomes extremely expensive and time consuming. Moreover, because of the length of time required for machine testing, the inventory of stockpiled photoreceptors waiting approval based on life testing of machines can reach unacceptably high levels. For example, a batch may consist of many rolls, with each roll yielding thousands of belts.
Another test method utilizes a stylus scanner such as that described by Z. D. Popovic et al., “Characterization of Microscopic Electrical Defects in Xerographic Photoreceptors”, Journal of Imaging Technology, vol. 17, No. 2, April/May, 1991, pp. 71-75. The stylus scanner applies a bias voltage to a shielded probe, which is immersed in silicone oil and is in contact with the photoreceptor surface. The silicone oil prevents electrical arcing and breakdown. Current flowing through the probe contains information about defects, and scanning speeds up to 6×6 mm2 in about 15 minutes were achieved. Although the stylus scanner is a highly reproducible tool which enabled some important discoveries about the nature of charge deficient spots, it has the basic shortcoming of low speed.
Many attempts have also been made in the past to reduce the time of scan by designing contactless probes. For example, a probe has been described in the literature and used for readout of xeroradiographic (X-ray) amorphous selenium plates, (see, e.g., W. Hillen, St. Rupp, U. Schieble, T. Zaengel, Proc. SPIE, Vol. 1090, Medical Imaging III, Image Formation, 296 (1989); W. Hillen, U. Schieble, T. Zaengel, Proc. SPIE, Vol. 914, Medical Imaging II, 253 (1988); U. Schieble, W. Hillen, T. Zaengel, Proc. SPIE, Vol. 914, Medical Imaging II, 253 (1988); and U. Schieble, T. Zaemge, Proc. SPIE, Vol. 626, Medicine XIV/PACS IV, 86 (1986)). These probes rely on reducing the distance of a probe to a photoreceptor surface in order to increase resolution of the measurements. The typical distance of the probe to the photoreceptor surface is 50-150 micrometers. In order to avoid air breakdown, the ground plane of a xeroradiographic plate is biased appropriately to provide approximately zero voltage difference between the probe and photoreceptor surface.
In U.S. Pat. Nos. 6,008,653 and 6,119,536, the contents of both of which are incorporated herein by reference in their entirety, a contactless system and method for scanning a photoreceptor surface is described. In U.S. Pat. No. 6,008,653, entitled CONTACTLESS SYSTEM FOR DETECTING MICRODEFECTS ON ELECTROSTATOGRAPHIC MEMBERS, a contactless process is disclosed for detecting surface potential charge patterns in an electrophotographic imaging member, including applying a constant voltage charge to an imaging surface of a photoreceptor, and biasing a capacitive scanner probe having an outer shield electrode to within about ±300 volts of the average surface potential of the imaging surface. The probe is maintained adjacent to and spaced from the imaging surface to form a parallel plate capacitor with a gas between the probe and the imaging surface. Relative movement is established between the probe and the imaging surface, maintaining a substantially constant distance between the probe and the imaging surface. The probe is synchronously biased and variations in surface potential are measured with the probe. The surface potential variations are compensated for variations in distance between the probe and the imaging surface, and the compensated voltage values are compared to a baseline voltage value to detect charge patterns in the imaging member.
The process described in U.S. Pat. No. 6,008,653 is implemented using a system for maintaining a substantially constant distance between the probe and the imaging surface. This system is described in U.S. Pat. No. 6,119,536, entitled CONSTANT DISTANCE SCANNER PROBE SYSTEM. While ideally the distance between the probe and the imaging surface is maintained constant while scanning the imaging surface, in reality small variations do occur. An algorithm is provided for compensating for variation in the distance between the probe and the imaging surface. The algorithm is based on compensation for a flat plate capacitor in which charge is uniformly distributed. However, defects such as CDSs are small points. The point-like nature of the CDSs affects the charge distribution to be non-uniform, and the distance compensation algorithm currently used is not sufficient in correcting for the non-uniform charge distribution caused by the point-like nature of CDSs on the imaging surface.
Thus, there is a need for a system and method for correcting for the non-uniform charge distribution caused by the point-like nature of CDSs on the chargeable surface in conjunction with a scan operation of the chargeable surface.